1. Field of the Invention
The object of this invention relates to the method of efficiently and reliably uniformly coated metal pipes, particularly those with comparatively small diameter of less than 20 mm, for the use as supply routes for various types of fluid, without giving cause to the occurrence of metal corrosion, or the blockage of the interior of the pipe through the use of corrosion resistant tin and tin based alloy coating, and moreover, even in cases where double walled pipes are employed, without giving cause to the occurrence of segregation of the joint section at the time of processing.
2. Description of Prior Arts
Hitherto, small diameter metal pipes used as car brake oil pipes or as supply routes for other fluids or gaseous fuels were double walled or single walled metal pipes copper plated either on the interior wall only or on both sides. These pipes were, to some extent, corrosion resistant, due to the application of copper plating. However, problems resided, for example, when used as brake oil pipes, the brake oil had the effect of melting out the copper and causing electric corrosion of, for example, the aluminum or aluminum based material of the parts connected to the said brake pipe. There resided, also, the problem of the gasoline being degenerated in the fuel supply pipe.
Because of this, the procedure of coating the interior of the pipe with tin or tin based alloy, which can prevent such harmful effects, has been employed. As one of such means, the method has been proposed of inserting into the metal pipe a metal wire of zinc, tin-zinc, or alloy based on the said substances, with melting point below 450.degree. C., and heating it in a reducing atmosphere gradually to 800.degree.-1100.degree. C. to carburize a part of the zinc substance in the fusible metal wire, and, thus, to form a plated layer by adhering the fusible metal wire to the interior wall of the metal pipe (Japanese Patent Publication No. 18745/1985).
However, by this method, particularly where the double walled pipe, wherein metal strips are brazed with copper, is employed, as can be seen by the X-Ray photograph shown in FIG. 12, due to the strength of the heat far higher than the melting point of the coating material, and the length of the holding time, the molten tin powder flows out to the copper layer at the seam and forms a brittle Su-Cu alloy layer deep into the joint section. Because of this, the problem of the segregation of the inner seam seriously weakening the pressure resistance of the metal pipe at the time of such pipe edge processing, as bulge, flare and spool, and the bending process after the coating treatment was observed that, while reducing atmosphere is used in the heat treatment furnace, generally, the oxygen in the air still remains, to some extent, in the pipe so that, during the process of gradually increasing the temperature of the pipe, it has the effect of forming a hard oxidated layer consecutively in the axial direction of the pipe. Because of this shell-like formed hard oxidated layer, even when heated beyond the melting point of the coating material, the molten coating material remains within the oxidated layer shell and cannot flow out to the interior wall of the pipe. It was also observed that the length of the heating furnace in the axial direction of the pipe was another factor in causing the said effect continuously in the longtitudinal direction. Moreover, by further heating, the said oxidated layer forming a shell, was torn in an instant by the molten substance, by pyrolysis, or by the steam pressure of the fusible wire, and the molten metal around this area flowing inside this shell flows out in a concentrated area. Because this molten metal, having attained a high fluidity due to the activation and low adhesivity caused by heating to a high degree, and also to the metal steam pressure, flows out with force and with consistency, Sn-Fe, Zn-Fe are formed at the interior wall of the metal pipe, near the aperture of the outflow, iron readily melts out, and the base metal is corroded in a concentrated localized are (FIG. 13), causing the phenomenon generally termed metal corrosion. The pipe wall becomes thin and the phenomenon seriously weakening such mechanical strength as the vibration resistance and pressure resistance appears intermittently in the axial direction of the pipe. Also, in extreme cases, there exists the problem of leakage through permeation, and, particularly in the cases employing materials of some length having a minor diameter of less than 4 mm .phi., there also exists the problem of blockage of the pipe interior through the concentrated outflow of the said molten metal. Furthermore, deposits occur near the localized are of the outflow (FIG. 11), while in other parts separated from this area, coating is hardly formed at all (FIG. 9), and there existed, in particular, the problem of non-uniform coating formation in the longitudinal direction.